The present disclosure is directed to quantum computation. More particularly, the present disclosure relates to systems and methods for storing and processing quantum information.
In the field of quantum computation, the performance of quantum bits (“qubits”) has advanced rapidly in recent years, with a variety of qubit implementations proving to be promising candidates for scalable computing architectures. In contrast with classical computational methods, where data is manipulated and stored in the form of well-defined binary states, or bits, quantum computation takes advantage of the quantum mechanical, probabilistic nature of quantum information. Quantum systems characterized by quantized energy levels can represent a superposition of multiple quantum states.
In general, a qubit can encode quantum information using a simple two-level system whose state can be represented as a vector in a two-dimensional complex Hilbert space. In some approaches, such two-level systems may be constructed using a semiconductor-based quantum dot system, as well as other physical systems. Such implementations are advantageous due to their scalability and ease of integration with present semiconductor-based electronics technologies. In general, quantum dots are artificially structured systems that can be filled with electrons or holes which may become trapped in three dimensions using controllable potential barriers generated by various device configurations and/or electrical gating. The confined electrons or holes can then form localized bound states with discrete energy levels, similar to the quantum states of atoms and molecules. The wavefunctions describing these states may then be utilized to establish the two-level system. Specifically, if the spatial part of an electron wavefunction is used, a charge qubit is achieved, with the spatial wavefunction defining the electron charge distribution. On the other hand, if the spin portion of the wavefunction is used, a spin qubit is produced.
A charge qubit can be implemented using a double-dot configuration having a single excess electron at the highest occupation level localized on one of the dots. However, charge qubits have high decoherence, resulting in the loss of information stored in the qubit. Specifically, the motion of charged defects in the device gives rise to time-varying electric fields causing fluctuations in the detuning, which is the energy difference between the two charge states of the qubit. Such fluctuations reduce the coherence time of the charge qubit, limiting computational applications. In some aspects, the charge qubit coherence may be improved by operating at a “sweet spot,” defined as a special value of the detuning where the derivative of the energy difference between the qubit states as a function of detuning is zero. However, so far it has not been possible to achieve high fidelity operations in a charge qubit by exploiting a single sweet spot. In particular, universal qubit control requires the ability to perform rotations about a second axis, and for dc pulsed gates, this second rotation axis needs to be implemented away from the sweet spot, yielding low gate fidelities. In principle, ac gating can be used to perform a universal set of qubit rotations without leaving the sweet spot. However, in practice it is difficult to perform operations with fidelities high enough for quantum information processing applications since noise moves the qubit away from the sweet spot.
Spin qubits can be sufficiently decoupled from their environments, thus providing relatively long quantum information lifetimes for performing computation. However, manipulation of electron spins in quantum dots requires precise control over the magnetic properties of the device and the ability to generate fast-pulse localized magnetic fields. Also, spin qubits are more difficult to couple to external circuitry, and often necessitate use of various spin-charge conversion techniques. By contrast, charge qubits can easily be coupled to external circuitry, facilitating control and measurement. In addition, charge qubits can be easily integrated with present semiconductor technologies, and lend themselves well to scalability due to ease of spatial selectivity addressing individual qubits in a multi-qubit quantum computer architecture. In addition, charge qubits can be manipulated quickly up to gigahertz frequencies. However, charge qubits are susceptible to environmental noise that is intrinsic to the materials and geometries used, and suffer from relatively poor gate fidelities for the same reasons. As such, spin qubits are often considered to be leading candidates in the realization of semiconductor quantum dot-based quantum computers.
In light of the above, there is a need for systems and methods for quantum computing based on charge qubit implementations that are amenable to realistic computing architectures.